The invention relates to a method for the compensation control of offset voltages in the baseband of a radio receiving circuit which is integrated in a circuit module and operates using the time division multiplex method (TDMA=time division multiple access) with reception time slots, using a sample operational amplifier which taps off the baseband output signal and, via a sampling switch of a sample-and-hold circuit, which switch is activated by an OCE (offset compensation enable) control signal before each reception time slot for carrying out an offset compensation operation for a defined period of time, charges or discharges an externally fitted capacitor depending on the polarity of the offset voltage, the respective applied voltage of which capacitor is compared with a reference voltage internally in the module by means of a differential amplifier and provides for the generation of a compensation current which counteracts the offset voltage. The invention also relates to a circuit for carrying out the method.
In an integrated radio receiver or radio transceiver circuit module, in the reception path a reception signal (useful signal) present at radio frequency at the antenna is converted firstly into an intermediate frequency (IF) and then, by means of a demodulator (mixer), into baseband (BB).
Offset voltages can occur in this reception path in dependence on production characteristics. These offset voltages are amplified in the same way as the useful signal by the amplifiers in the reception path. If an offset voltage of, say, 20 mV is present at the input of a 16 dB amplifier, then an offset voltage of 126 mV is produced at the output of the amplifier. This high offset voltage leads to the useful signal being limited, since the dynamic range of this amplifier and all subsequent amplifiers in the reception path is reduced by the offset voltage.
In all high-frequency circuit sections (RE and IF), the offset voltage can be eliminated at suitable locations by means of capacitive coupling. However, in the baseband, in which the signal frequencies are about 100 kHz, capacitive coupling is no longer possible, since very large capacitances would have to be used in the integrated circuit module for the capacitive coupling. However, such large capacitances require very long charging times when the circuit module is switched on and take up a considerable chip area or cannot be realized at all on an integrated circuit module.
European Patent application EP 0 693 823 discloses compensating for the offset voltages present in the baseband with the aid of a sample-and-hold circuit.
As is shown in detail by a schematic illustration of such compensation in FIG. 1, to that end the differential baseband signal, which is obtained from the IF signal IF via a demodulator 1 operated with a local oscillator frequency LO and is filtered downstream of a buffer amplifier 2 by means of a low-pass filter 3. The signal is then tapped off internally downstream of a baseband regulating amplifier 4 at the baseband output IR, IRX, which is connected to a downstream baseband processor 5, and is fed to a sample operational amplifier 6.
Via a controlled sampling switch 7 of the sample-and-hold circuit, the sample operational amplifier 6 charges or discharges an externally fitted capacitor C via an output CSHI depending on the polarity of the offset voltage Voffset. The capacitor voltage is fed to a differential amplifier 8, where it is compared with a reference voltage VRef.
Compensation currents which counteract the offset voltage Voffset are generated at the output of the differential amplifier 8. The offset compensation operation is carried out before each reception time slot of the radio receiver operated using time division multiplex (TDMA). To that end, the sampling switch 7 is controlled by means of a so-called OCE (offset compensation enable) control signal OCE in the prior art circuit.
In the prior art, in the case of the pulsed control signal OCE for controlling the compensation operation, the control signal being generated externally in the baseband processor 5, the pulse width is likewise enlarged in the case of large offset voltages.
The relationships in this regard are explained in detail below. Such compensation of the offset voltage Voffset in the baseband using a sample-and-hold circuit in accordance with FIG. 1 can be realized in an integrated receiver circuit modulexe2x80x94designated by 9xe2x80x94for example, for portable GSM mobile radio transceivers.
The offset voltage compensation operation lasts for a specific period of time, for example 50 xcexcs in the case of a receiver in the GSM system with its customary TDMA time frames. The offset must have been compensated for in this short period of time. The period of time taken to compensate for an offset that is present depends on the magnitude of the offset voltage, the magnitude of the capacitance of the external capacitor and the magnitude of the charging current of the sample operational amplifier.
The magnitude of the capacitance of the external capacitor and of the charging current must be set such that a typical offset voltage can precisely be corrected in the short time period of, for instance, 50 xcexcs. The correction or compensation operation could be shortened by reducing the magnitude of the capacitance of the external capacitor and by increasing the charging current. However, this can lead to the regulation becoming unstable and oscillations being produced.
It also happens that the external compensation capacitor is discharged, so that very large offset voltages then occur. This is the case particularly when the radio receiver is switched on for the first time, has been switched off for a relatively long time, or when the capacitor has become discharged for some other reason. In this case, the short available period of time of e.g. 50 xcexcs is insufficient for compensation of the disturbing offset, since the capacitor would have to be charged considerably more than is possible in this short period of time.
For the reasons mentioned above (instability, oscillation tendency), it is not possible to accelerate the charging by appropriate dimensioning of the sample-and-hold circuit.
To date, therefore, the offset compensation operation has been prolonged. This requires additional computation complexity on the part of the baseband processor. Moreover, on account of the longer duration of the compensation, e.g. about 200 xcexcs instead of 50 xcexcs, the subsequent reception time slot can no longer be utilized.
If the OCE control signal is not generated by the baseband processor, but in the receiver circuit module itself, then the duration of the respective offset compensation operation is established at a fixed duration of e.g. 50 xcexcs. In this case, therefore, a plurality of compensation cycles would be necessary in order to compensate for a relatively large offset.
The object of the invention is to provide a method and circuit for compensation control of offset voltages of a radio receiving circuit integrated in circuit module which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of t kind, and which make it possible to carry out the offset voltage compensation in the baseband in every instance, tha is to say even when relatively large offset voltages are present during a fixed, defined compensation duration of e. 50 xcexcs in the radio receiver or transceiver chip, without additional computer power being necessary in the baseband processor and without disturbing instability or oscillation tendency of the sample-and-hold circuit being brought about.
With the above and other objects in view there is provided, in accordance with the invention, a method of compensating for offset voltages in a baseband of a radio receiving circuit integrated in a circuit module and operating in a time division multiplex method with reception time slots, the method which comprises the following steps:
feeding a baseband output signal to a controllable sample operational amplifier;
selectively charging or discharging an external capacitor depending on a polarity of an offset voltage, by activating a sampling switch of a sample-and-hold circuit with an offset compensation enable control signal before each reception time slot for a defined period of time;
comparing a voltage of the capacitor with a reference voltage internally in the module with a differential amplifier;
generating a compensation current counteracting the offset voltage;
determining a charge state of the capacitor internally in the module, and, when the capacitor charge is substantially below a typical charge state, increasing the charging current to the capacitor with the controllable sample operational amplifier, and reducing the charging current back to a normal charging current when the charge state of the capacitor attains a charge in the typical charge state range.
In other words, the charge state of the externally fitted compensation capacitor is evaluated on-chip. If the capacitor is discharged or if the instantaneous charge state is far below the typical charge state, then, on the basis of the result obtained in the charge state evaluation, the charging current is increased in the sample operational amplifier. In accordance with one advantageous development of the invention, such a process of increasing the charging current in the sample operational amplifier is performed by switching on an additional current source. With the aid of the increased charging current, the capacitor is now charged significantly faster. When the charge state of the capacitor finally attains the order of magnitude of the typical charge state, the increased charging current generation in the sample operational amplifier is switched back again. The consequence of this is that the remaining compensation of the xe2x80x94now reducedxe2x80x94offset proceeds at the normal speed. The increased charging current is switched back in good time to ensure that the sample-and-hold circuit does not become unstable and, moreover, does not start to oscillate.
The method according to the invention for accelerating the offset voltage compensation as a function of the charge state of the compensation capacitors allows the principle of offset voltage compensation by means of a sample-and-hold circuits also to be used in receiver and transceiver circuits in which a rigid duration of 50 xcexcs, for example, is defined for the offset compensation operation. The method according to the invention enables arbitrarily occurring offset voltages to be compensated for in the predetermined time, without the regulation entering an unstable state or starting to oscillate.
The invention is thus essentially to be seen in the evaluation of the charge state of the compensation capacitor on-chip. The result of this evaluation is then used to match the charging current of the sample operational amplifier to the respective charge state of the compensation capacitor. A minimal offset compensation time with regulation that remains stable is thus achieved.
With the above and other objects in view, there is also provided an integrated circuit module for carrying out the above-outlined method. The circuit comprises:
a charging current evaluation circuit having an input connected to a module-internal feed to an external capacitor and an output, the evaluation circuit having a differential amplifier connected to compare a charge state of the capacitor with a reference charge value;
a controllable sample operational amplifier provided with an additional current source connected via a control signal line to the output of the evaluation circuit and activated via the output if a differential value, ascertained in the differential amplifier, formed from the charge state value of the capacitor and the reference charge value exceeds a defined value, and deactivated if the differential value falls below another defined value.
In other words, the circuit for carrying out the method is characterized in that a charging current evaluation circuit is provided on the integrated circuit module, one input of which evaluation circuit is connected to the module-internal feed to the externally fitted capacitor, in that the evaluation circuit has a differential amplifier, which compares the charge state of the capacitor with the reference charge value, and in that the sample operational amplifier is provided with an additional current source which is connected via a control signal line to the output of the evaluation circuit and is activated via the latter if the differential valuexe2x80x94ascertained in the differential amplifierxe2x80x94formed from the charge state value of the capacitor and the reference charge value exceeds a defined value, and is deactivated again if said differential value falls below another defined value again.
The method and the above-specified circuit for carrying out this method can be used particularly advantageously in a small portable mobile radio transceiver (mobile phone) which is used in a mobile radio system operating using time division multiplex (TDMA), for example in a GSM or PCN/PCS mobile radio system.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method and circuit for compensation control of offset voltages of a radio receiving circuit integrated in a circuit module, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.